Lithographic reticles are employed for patterning substrate areas to be etched, such as to create a gate electrode of transistor. A “reticle,” as shown in FIG. 1 is a hard copy of the pattern to be imaged created in a thin layer of an opaque material, such as chrome 120, deposited on a glass or quartz reticle plate 115. The reticle may be used to image the pattern onto a semiconductor substrate. Alternatively, the reticle may be used to produce another reticle or photomask.
Pellicles are employed on the patterned (chrome) side of the mask to prevent propagation of particle-related defects into patterns imaged onto the substrate. Generally, a pellicle includes a pellicle membrane 110, transparent to the lithographic radiation, which covers the chrome 120 at an elevation defined by a pellicle frame 105 to keep particles out of depth of focus. Particles which land on the pellicle membrane 110 or on the back side of the mask 125 contribute little to the patterning process since they are not within the object plane of the imaging system. Pellicle membranes 110 may be either of “soft” polymer-based materials on the order of 1 um thick or “hard” silica-based materials. Pellicle frame 105 is typically a metal, such as anodized aluminum, which can be relatively inexpensively machined and kept clean during use. The pellicle frame 105 includes a perimeter of walls which are attached by an adhesive to a chrome side of the reticle plate 115. Typically, the frame 105 is a solid forming a simple rectangular cross-sectional area, as depicted.
The lithographic process may be characterized with an amount of pattern displacement from the desired location, referred to as a registration error. As depicted in FIG. 1, the mounting of the pellicle frame 105 can alter the flatness of plate 115 by exerting mechanical stresses on the mask. This non-flatness induced by the mounting of the pellicle frame 105 to reticle plate 115 may induce registration error by changing where a pattern in chrome 120 is imaged onto a substrate. For example, articles have described how pellicles can affect the shape of the mask as a function of initial pellicle flatness and temperature change (Cotte et al., Experimental and Numerical studies of the Effects of Materials and Attachment Conditions on Pellicle-Induced Distortions in Advanced Photomasks, SPIE Vol. 4754, pp. 579-588 (2002)). While some amount of symmetrical distortion of reticle plate 115 may be cancelled with correction algorithms employed during substrate imaging, flatness of the pellicle frame 105 as well as the mounting process may induce asymmetrical distortion, whereby plate 115 warps in one dimension by a first amount and warps in a second dimension by a second amount. Furthermore, unless the distortion induced is highly repeatable, plate-specific correction algorithms require accurate measurement of plate distortion.
As the amount of registration error tolerable generally scales with feature resolution, the magnitude of registration error induced by the pellicle has become a critical path in the continued scaling of photo lithography.